Kathryn Felicia Day (born June 28, 1979) is an American actress, writer, and web series creator. She is the creator and star of the web series The Guild (2007–2013), a show loosely based on her life as a gamer. She also wrote and starred in the Dragon Age web series Dragon Age: Redemption (2011). She is a founder of the online media company Geek & Sundry, best known for hosting the show Critical Role between 2015 and 2019. Day was a member of the board of directors of the International Academy of Web Television from December 2009 until August 2012.

On television, Day has played Vi in the series Buffy the Vampire Slayer (2003) and Dr. Holly Marten in Eureka (2011), and had a recurring role as Charlie Bradbury on Supernatural (2012–2015, 2018–2020). She has also acted in movies such as Bring It On Again (2004), as well as the Internet musical Dr. Horrible's Sing-Along Blog (2008). In April 2017, she began appearing as Kinga Forrester in Mystery Science Theater 3000.